Germanium dioxide (GeO.sub.2) is the dopant chiefly used in CVD (chemical vapor deposition) techniques for fabricating the optical fiber core, both in case of inside vapor deposition processes (IVPO) in which a tube is internally coated and then collapsed, and in case of outside processes (OVPO) in which a rod is coated.
In fact, GeO.sub.2 gives with silica in the coating a binary network having a stable vitreous state. The halide source, germanium tetrachloride (GeCl.sub.4), from which the GeO.sub.2 can be obtained by oxidation is particularly suited for use in CVD techniques, since, at room temperature it is an easily vaporable liquid (melting temperature T.sub.m =-49.5.degree. C.; boiling temperature T.sub.b =84.degree. C.).
The optical properties of germanium dioxide are particularly interesting: practically no dispersion at a wavelength of 1.74 .mu.m; and an infrared absorption peak due to melecular vibration of Ge--O bond centered at a wavelength of about 12 .mu.m.
The latter property prevents modifications of the silica spectral-attenuation curve, which has an infrared absorption peak for the molecular vibration of Si--O bond centered at a slightly lesser wavelength (9.1 .mu.m).
For these reasons germanium dioxide is nowadays the most widely used compound in optical fiber technology and has generally been the only one used for fabricating the core of silica-based optical fibers.
It presents, however, two disadvantages:
(i) high cost of raw material; and
(ii) a Rayleigh scattering coefficient higher than that of pure silica, whose value is about 0.6 dB/km/ m.sup.4.
The effect of germanium dioxide in the binary lattice structure SiO.sub.2 --GeO.sub.2 is that of increasing the scattering coefficient value in proportion to the concentration of the dopant present in the network.
That is detrimental to performance because of a significant increase in minimum attenuation values.
Alumina (Al.sub.2 O.sub.3) is an attractive alternative to GeO.sub.2 ; in fact, in addition to having all the advantages of germanium dioxide, it presents the following characteristics:
(a) a Rayleigh scattering coefficient less than that of silica;
(b) lower cost of raw material; and
(c) high melting teperature.
It is of interest to underline the fact that a scattering coefficient lower than that of silica can allow the lowest attenuation levels to be reached for silica-based vitreous networks.
More particularly with vitreous networks using a SiO.sub.2 --Al.sub.2 O.sub.3 core system a minimum attenuation value lower than that of silica can be obtained; for silica this value is equal to 0.12 dB/km in the wavelength range of about 1.56 m.
The high melting temperature is particularly advantageous.
The melting temperature of alumina (2045.degree.) is higher than those of silica (1703.degree. C.) and of germanium oxide (1086.degree. C.). The physical properties of a SiO.sub.2 --Al.sub.2 O.sub.3 lattice structure are hence closer to those of an SiO.sub.2 lattice structure than those of an SiO.sub.2 --GeO.sub.2 lattice structure.
In addition, the presence of a compound with higher melting point prevents the dopant diffusion towards the periphery during the preform collapsing step when the system is fabricated using an inside position method.
As a consequence, alumina-doped silica fibers fabricated by the MCVD technique do not present any dip (i.e. central refractive index decrease). This is a typical anomaly in the profile of germanium-dioxide doped silica fibers, fabricated by the same technique.
The main disadvantage preventing alumina from being industrially utilized as a dopant theretofore is that satisfactory liquid or gaseous compounds at room temperature, to be used as aluminum vehicles and hence suited to CVD techniques, are difficult to find.
Aluminum halides are solid at room temperature and have rather high boiling temperatures. For example, AlF.sub.3 sublimes at 1291.degree., AlCl.sub.3 sublimes at 178.degree. C., AlBr.sub.3 melts at 97.degree. C. and boils at 263.degree. C., AlI.sub.3 melts at 191.degree. C. and boils at 360.degree. C. The use of CVD technique with such raw materials requires reactant mixing and vaporization lines thermostated at high temperature. That entails implementing difficulties and does not assure pollution-free synthesis products.
Besides, solid compounds at room temperature are more difficult to purify by comparison with liquid or gaseous compounds; hence they can contain residual impurities detrimental to optical properties.
The use of AlCl.sub.3 as a basic aluminum vehicle has been already suggested in the paper entitled "Fabrication of Low-Loss Al.sub.2 O.sub.3 doped silica fibers" by Y. Ohmori et al. Electronics letters, Sept. 2, 1982, Vol. 18, No. 18, without a solution of the problem.
A method of fabricating alumina-doped silica fibers has been reported in the previously mentioned Italian patent application No. 68135-A/84, filed on Nov. 13, 1984. In the system described there, silica and dopant are obtained by a reaction between gaseous chemical compounds, dopant being obtained by the reaction between oxygen and an organometallic compound of the Al(C.sub..alpha. H.sub..beta.).zeta. or of AlCl(C.sub..alpha. H.sub..beta.).psi. type, where .alpha., .beta., .zeta., .psi. are respectively coefficients of presence in molecules, of the atoms C, H and of the hydrocarbon respectively. The coefficients of presence are here defined as the number of atoms or moieties over molecule.
These organometallic compounds are liquid at room temperature; hence this method allows silica to be doped with alumina by using a Chemical-Vapor-Deposition technique (CVD) without requiring the use of vaporization and mixing lines thermostated at high temperature. The obtained optical fibers present low attenuation and are not affected with the dip.
However organometallic compounds are highly reactive with oxygen even at low temperature.
This is a drawback because the reaction, besides occurring spontaneously, proceeds rapidly to completion at high efficiency, thereby making control of the deposition phase of silica and alumina difficult.
In addition this high aluminum alkyl reactivity with oxygen presents serious problems concerning plant safety.